Whooping cough

This is a series of occasional articles highlighting conditions that may be commoner than many doctors realise or may be missed at first presentation. The series advisers are Anthony Harnden, university lecturer in general practice, Department of Primary Health Care, University of Oxford, and Richard Lehman, general practitioner, Banbury. If you would like to suggest a topic for this series please email us (easilymissed.bmj@bmjgroup.com)


Why is it missed?
In the post-vaccination era, whooping cough is under-recognised in primary care as the incidence is incorrectly thought to be low. The classic clinical features of whooping cough, such as an inspiratory "whoop" (listen on bmj.com), may be attenuated in older children and adults who have been immunised. 1 Moreover, many doctors may not be aware that there is a simple diagnostic serological test.

Why does this matter?
A persistent cough without explanation can cause distress and anxiety, and the patient may be subject to inappropriate investigations, treatment, and referral. 5 Secondly, early diagnosis and treatment with erythromycin can prevent the patient transmitting their infection within and outside the household. 6 This may be especially important in young infants, who may have severe complications such as respiratory failure and death. 7 Thirdly, notification of all cases of whooping cough by primary care clinicians and enhanced surveillance of laboratory proved cases would give a better estimate of the efficacy of the vaccine and of the burden of disease in the community.

Clinical features
Acute cough persisting for more than two weeks, without the characteristic inspiratory whoop, may be the only clinical symptom of whooping cough. Initial symptoms, lasting for up to two weeks, mimic a simple upper respiratory tract infection. This is the most infectious period. Paroxysmal episodes of coughing may continue for up to six weeks but can recur with further respiratory infections. In China whooping cough is referred to as the one hundred day cough. In all age groups, irrespective of immunisation status, the cough lasts an average of three months.

Investigations
Bordetella pertussis is a difficult organism to culture without a correctly collected pernasal swab or nasopharyngeal aspirate; neither of these samples is easy to collect in primary care. The sensitivity of culture falls from 15-45% to 0% during the first three weeks of the cough, and sensitivity may be further reduced by antibiotic treatment, previous immunisation, and transport of the specimen. 8 Serology is the recommended diagnostic blood test in primary care and is routinely used in many countries: a single raised titre of antipertussis toxin IgG has a sensitivity of 76% and a specificity of 99% for the diagnosis of whooping cough across all age groups. 9 More recently, assays of antipertussis toxin IgG levels in oral fluid have been validated; they are quick and simple to use in primary care and have recently been made available in the UK. 10

How is it managed?
Whooping cough should be notified. Treatment may not affect outcome for the patient, but erythromycin within 21 days of onset of symptoms reduces the period of infectivity and may prevent transmission to household members. 6 A seven day course is sufficient. Prophylaxis with erythromycin should be offered to everyone in households with a vulnerable infant who may be unimmunised or partly immunised. 6 In infants the illness may be severe and require prompt referral to secondary care.
I thank Doug Jenkinson for permission to use an audio clip from his website and for his work in promoting the clinical recognition of whooping cough in primary care.